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To investigate the effect of a cyclic structure on the reactivity of conjugated dienes in cationic polymerization, a cyclic diene 3-methylenecyclohexene (1) and the corresponding linear diene 2-methyl-l,3-pentadiene (2) were synthesized and polymerized. Cationic polymerizations and copolymerizations showed that 1 was more reactive than 2. The conjugated double bonds in 1 were proved to be less resonance-stabilized than that in 2. However, the conjugate cation 1 was found to be more stable than that of 2. The combination of these two factors made the cyclic diene more reactive than the linear diene. Cationic polymerizations of 2 were fast and non-stationary, giving low molecular weight polymers. The polymerizations proceeded mainly by the 1,4 route and to a smaller extent by the 1,2 route. A brief discussion on the stereochemistry in some electrophilic addition reactions of methyl-substituted butadienes is also given. ZUSAMMENFASSUNG:Um die Wirkung einer cyclischen Struktur auf die Reaktivitat konjugierter Diene bei der kationischen Polymerisation zu untersuchen, wurden ein cyclisches Dien, 3-Methylencyclohexen (l), und das entsprechende lineare Dien, 2-Methyl-l,3-pentadien (2), dargestellt und polymerisiert. Kationische Polymerisationen und Copolymerisationen zeigten, daD 1 reaktiver als 2 ist. Es wurde nachgewiesen, daB die konjugierten Doppelbindungen in 1 weniger resonanzstabilisiert sind als diejenigen in 2. Es wurde jedoch gefunden, daB das konjugierte Kation von 1 stabiler ist als das von 2. Die Kombination dieser beiden Faktoren verursacht bei cyclischen Dienen eine hohere Reaktivitat als bei linearen Dienen. Kationische Polymerisationen von 2 waren schnell und nicht-stationar und fuhrten zu Polymeren mit niedrigem Molekulargewicht. Die Polymerisationen verliefen hauptsachlich nach dem 1,4-Prinzip und zu einem geringeren AusmaD nach dem L2-Prinzip. Die Stereochemie bei einigen elektrophilen Additionen methylsubstituierter Butadiene wird ebenfalls kurz diskutiert.
To investigate the effect of a cyclic structure on the reactivity of conjugated dienes in cationic polymerization, a cyclic diene 3-methylenecyclohexene (1) and the corresponding linear diene 2-methyl-l,3-pentadiene (2) were synthesized and polymerized. Cationic polymerizations and copolymerizations showed that 1 was more reactive than 2. The conjugated double bonds in 1 were proved to be less resonance-stabilized than that in 2. However, the conjugate cation 1 was found to be more stable than that of 2. The combination of these two factors made the cyclic diene more reactive than the linear diene. Cationic polymerizations of 2 were fast and non-stationary, giving low molecular weight polymers. The polymerizations proceeded mainly by the 1,4 route and to a smaller extent by the 1,2 route. A brief discussion on the stereochemistry in some electrophilic addition reactions of methyl-substituted butadienes is also given. ZUSAMMENFASSUNG:Um die Wirkung einer cyclischen Struktur auf die Reaktivitat konjugierter Diene bei der kationischen Polymerisation zu untersuchen, wurden ein cyclisches Dien, 3-Methylencyclohexen (l), und das entsprechende lineare Dien, 2-Methyl-l,3-pentadien (2), dargestellt und polymerisiert. Kationische Polymerisationen und Copolymerisationen zeigten, daD 1 reaktiver als 2 ist. Es wurde nachgewiesen, daB die konjugierten Doppelbindungen in 1 weniger resonanzstabilisiert sind als diejenigen in 2. Es wurde jedoch gefunden, daB das konjugierte Kation von 1 stabiler ist als das von 2. Die Kombination dieser beiden Faktoren verursacht bei cyclischen Dienen eine hohere Reaktivitat als bei linearen Dienen. Kationische Polymerisationen von 2 waren schnell und nicht-stationar und fuhrten zu Polymeren mit niedrigem Molekulargewicht. Die Polymerisationen verliefen hauptsachlich nach dem 1,4-Prinzip und zu einem geringeren AusmaD nach dem L2-Prinzip. Die Stereochemie bei einigen elektrophilen Additionen methylsubstituierter Butadiene wird ebenfalls kurz diskutiert.
Abstract1‐Vinylcyclohexene (VCH), which has one of the double bonds in the ring and the other outside the ring, was synthesized and polymerized by cationic catalysts. The reactivity of VCH was very large in the polymerizations catalyzed by boron trifluoride etherate (BF3OEt2) and stannic chloride–trichloroacetic acid complex. Similar to other cyclic dienes, the polymerization of VCH was a nonstationary reaction having a very fast initiation step. The polymerization proceeded by either a 1,2‐ or a 1,4‐propagation mode in which vinyl group was always involved. Particularly when BF3OEt2 was used as a catalyst, an intramolecular proton or an intramolecular hydride ion transfer reaction took place, resulting in the formation of methyl groups in the polymer. The degree of polymerization of polymer formed was about 10. This indicates the preponderance of monomer transfer reaction. To investigate the reason for the high reactivity of cyclic dienes, cationic copolymerizations of VCH and 3‐methyl‐cis/trans‐1,3‐pentadiene (cis/trans‐MPD) was carried out. The relative reactivity of monomers decreased in the order VCH > trans‐MPD > cis‐MPD. On the other hand, the resonance stabilization of monomers decreased in the order VCH > trans‐MPD > cis‐MPD. Therefore, it could be considered that the monomer reactivity is mainly determined by the stability of carbonium ion intermediate. The relative stability of carbonium ion must be VCH > trans‐MPD > cis‐MPD. Thus the influence of the conformation of ion on its stability was clearly demonstrated.
Synopsisl-Methyl-3-methylenecyclobutene (MMCB) and 1,2-dimethylenecyclobutane (DMCB) copolymerized readily with isobutene with aluminum chloride as initiator in methyl chloride solution at temperatures from -95 to -78OC. No polymers were obtained with methylenecyclobutene (MCB) under similar conditions. The copolymerization of MMCB with isobutene took place through a 1,5-addition reaction while that of DMCB through both 1,2-and l,4-addition reactions. Large amounts of gel were present in the copolymers obtained from DMCB if the reaction was carried to high conversion. The commonly observed effects of dienes (i.e., rate retardation and molecular weight depression) on cationic copolymerization reactions were observed but to a much higher degree with these small ring dienes. The thermal crosslinking behavior of the resulting copolymers was investigated. In conjunction with the copolymerization studies, homopolymers of MMCB, DMCB, and 3,3-dimethyl-l-isopropylidene-2-methylenecyclobutane (IMCB) were prepared and their chemical structures examined.
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